1. Field of the Invention
The present invention relates to a system that performs encrypted transfer of data such as digitized text, audio, video, or programs via a transfer medium or recording medium, and in particular relates to a technique for encrypted transfer from a single transmission apparatus to a plurality of reception apparatuses using a secret key.
2. Description of the Related Art
In recent years, secret key encrypted communication systems have been developed which encrypt data such as digitized text, audio, video, or programs using a secret key, before transferring the data using a transfer medium or recording the data onto a recording medium for later reproduction. In such systems, both transmission apparatuses and reception apparatuses perform encryption and decryption using a predetermined secret key with which they are both provided.
When both transmission apparatuses and reception apparatuses are provided with a plurality of secret keys, it is necessary to specify which secret key is to be used before communication is commenced. This is performed to ensure that the reception apparatus will be able to decrypt the cryptogram transmitted by the transmission apparatus.
Conventional encrypted communication systems which are constructed so that one transmission apparatus supplies information to a plurality of reception apparatuses (hereinafter referred to as a "one-to-many" system) suffer from a major drawback in that when the secret key is leaked, it becomes necessary to set a new secret key in each reception apparatus, which requires the significant burden of changing the system construction itself.
As one example, when a secret key used by one broadcast station and one hundred reception apparatuses that receive programs transmitted by the broadcast station is leaked, it becomes necessary to change the secret key stored by the broadcast station and all one hundred reception apparatuses. This means that when the communication between one transmission apparatus and one reception apparatus is intercepted and the secret key decoded, this affects not just the two apparatuses involved in the intercepted communication, but every reception apparatus that is equipped with the same secret key.